Efforts have been underway for many years to develop implantable pressure transducers and sensors for temporary or chronic use in a body organ or vessel and systems for recording absolute pressure. Many different designs and operating systems have been proposed and placed into temporary or chronic use with patients. Indwelling pressure sensors for temporary use of a few days or weeks are available, and many designs of chronically or permanently implantable pressure sensors have been placed in clinical use.
U.S. Pat. No. 4,023,562 describes a pressure transducer comprising a piezoresistive bridge of four, orthogonally disposed, semiconductor strain gauges formed interiorly on a single crystal silicon diaphragm area of a silicon base. A protective silicon cover is bonded to the base around the periphery of the diaphragm area to form a sealed, evacuated chamber. Deflection of the diaphragm due to ambient pressure changes is detected by the changes in resistance of the strain gauges. Because the change in resistance is so small, a high current is required to detect the voltage change due to the resistance change. The high current requirements render the piezoresistive bridge unsuitable for long term use with an implanted power source. High gain amplifiers that are subject to drift over time are also required to amplify the resistance-related voltage change.
Other semiconductor pressure transducers employ CMOS IC technology in the fabrication of pressure responsive silicon diaphragm bearing capacitive plates that are spaced from stationary plates. The change in capacitance due to pressure waves acting on the diaphragm is measured, typically through a bridge circuit, as disclosed, for example, in the article "A Design of Capacitive Pressure Transducer" by Ko et al., in IEEE Proc. Symp. Biosensors, 1984, p. 32. Again, fabrication for long term implantation and stability is complicated. In addition, differential capacitive plate, fluid filled pressure transducers employing thin metal or ceramic diaphragms have also been proposed for large scale industrial process control applications as disclosed, for example, in the article, "A ceramic differential-pressure transducer" by Graeger et al., Philips Tech. Rev., 43:4:86-93, February 1987. The large scale of such pressure transducers does not lend itself to miniaturization for chronic implantation.
Piezoelectric crystal or piezoresistive pressure transducers mounted at or near the distal tips of pacing leads, for pacing applications, or catheters for monitoring applications, are described in U.S. Pat. Nos. 4,407,296, 4,432,372, 4,485,813, 4,858,615, 4,967,755, and 5,324,326, and PCT Publication No. WO 94/13200, for example. The desirable characteristics and applications for patient use of such lead or catheter bearing, indwelling pressure sensors are described in these and other patents and the literature in the field. Generally, the piezoelectric or piezoresistive transducers have to be sealed hermetically from blood.
The '296 patent discloses sealing the piezoresistive bridge elements within an oil filled chamber having a deflectable diaphragm so that blood pressure fluctuations can deflect the diaphragm and be transmitted by the oil to the pressure transducer. In one embodiment disclosed therein, a sealed inner chamber at a calibrated pressure is provided on one side of the piezoresistive bridge transducer, and the oil bears against the other side of the transducer. The incompressible oil is admitted into the chamber through a fill tube in a vacuum filling process to fill the chamber between the transducer and the diaphragm. The fill tube is then crimped.
The crimping of the end of the fill tube is not necessarily sufficient to prevent leakage of the oil from the chamber. Such pressure sensor leads are intended to be chronically implanted in patients for years or decades. The function of the pressure transducer will be impaired if the oil leaks from the chamber and is displaced by air or body fluids over such a period of time.
Moreover, the configuration of the pressure transducer housing employed in the '296 patent locates the diaphragm perpendicular to the lead body axis and facing distally. This constitutes a bulky transducer module and prevents its use at a location more proximal to the distal end of the lead body.
Despite the considerable effort that has been expended in designing such pressure sensors, a need exists for a body implantable, durable, long-lived and low power consuming pressure sensor for accurately sensing absolute pressure waves in the body over many years and for deriving body temperature signals in a system for demodulating and storing the signals.